In MIMO (Multiple Input Multiple Output) transmission systems like wireless LAN (Local Area Network) or vectored DSL (Digital Subscriber Line), training sequences are required to estimate the transmission channels, e.g. properties of connections used for data transmission, and crosstalk channels (e.g. strength of crosstalk) between all transmitters and receivers, and calculate channel equalizer and crosstalk precompensator/canceller coefficients according to the channel estimation.
For high frequency wired transmission, such as used in modern DSL lines that serve the last distance of the fiber to the distribution point (FTTdp) networks, channel estimation is required to be fast due to fast system reconfigurations, but also requires high precision to achieve high data rates.
In wireless systems, short training pulses are used for channel estimation. This is required for such systems, because the channel quickly changes over time and the channel estimation must be repeated very often, practically for each transmission. The dimension of the MIMO channel is defined by the number of antennas at transmitter and receiver side, which is a small number, usually from two to four. By sending one pulse on each transmit antenna while all receivers are listening, a complete channel estimation is performed in a short time or a single pulse, creating only little overhead. The disadvantage of the method is low accuracy and only very small number of systems to address.
In vectored DSL, the channel is much more stable that the wireless channel, but channel estimation requires very high precision to cancel crosstalk and the number of disturbers is very high comparing to wireless (up to hundreds). Since a DSL wireline channel is rather static over time, in modern DSL (ITU-T G.993.5) Walsh-Hadamard (WH) sequences are used for FEXT channel estimation. To reduce transmission overhead, the WH sequences used for channel estimation are spread over time, but this increases the FEXT estimation time accordingly. A complete channel estimation sequence of a 512-pair cable binder may take as much as 30 s and even more, which is obviously inconvenient and even impractical if the system requires more frequent updates.
In FTTdp deployment, there are often network topologies where multiple distribution points (DPs) share the same cable binder. Therefore, the channel estimation method must be able to measure crosstalk between different, non co-located, distribution points.